The output power of an electric motor is critically dependant on accurate synchronisation of phase energisation and rotor position. Accordingly, the motor requires some means for accurately determining the position of the rotor. A relatively cheap solution is a Hall-effect sensor, which may be located at a slot opening between poles of the stator. However, owing to the effect of armature reaction, the sensor is sensitive to both the rotor field and the armature field. For a stator having asymmetric poles, the effect of armature reaction makes it particularly difficult to determine the position of the rotor with any confidence.